Field of the Invention
Embodiments of the present disclosure relate generally relates to position sensors and more particularly to an elongated or extended stroke position sensor comprising a waveguide and a magnet.
Discussion of Related Art
Most non-contacting position sensors are based on Hall or magnetostrictive technology. The effective maximum stroke length that these types of position sensors are able to accurately sense is limited to about 20 mm; beyond 20 mm, the ability to linearly sense the magnetic field strength limits the performance of the position sensor. In particular, the sensitivity and accuracy of the position sensor suffers. As such, more complex, and often more expensive, measurement systems are used to determine the position of objects having a stroke of greater than 20 mm. For example, in order to measure the position of a piston in a cylinder, which typically has a stroke greater than 20 mm, a magnetic sensor may be used. However, the configuration of the magnetic sensor may necessitate that the length of the magnet would be approximately the same length as the stroke. As such, the cylinder size would have to be increased to include such a sensor.
Some non-contact position sensors use a magnetostrictive magnet-waveguide to measure position. For example, a circular magnet is positioned about a magnetostrictive waveguide. The position of the circular magnet is used to identify the position to be measured. An electric wire extends the length of the waveguide. A pulse of current is placed upon the wire, which creates a second magnetic field. The second magnetic field interferes with the magnetic field created by the circular magnet, thereby creating a torque pulse located at the position of the circular magnet. The time it takes for the torque pulse to move to a sensor head is correlated with the position of the circular magnet. An example of such a system is described in U.S. Pat. No. 5,717,330, which is incorporated herein by reference. As will be appreciated, the torque pulse moves down the waveguide at the speed of sound. While this is relatively fast, if the magnet is placed on a fast-moving machine head measurement inaccuracies may be produced.
Additionally, some non-contact position sensors use other types of waveguides. However, these other types of waveguides may rely on interactions with the local environment to measure position. For example, U.S. Pat. No. 5,249,463, which is incorporated herein by reference, teaches a waveguide used to determine the level of a liquid. However, as will be appreciated, the accuracy of such position sensors will be affected by exposure to the environment (e.g., other liquids, oils, etc.).
Thus there is a need for a non-contact position sensor configured to measure objects having a stroke greater than 20 mm, which may move at relatively fast speeds, and which may be operated in harsh environments (e.g., exposed to liquids, oils, etc.).